When Cross Compiling for Android this variable contains the absolute path prefixing the toolchain GNU compiler and its binutils.
See also CMAKE_<LANG>_ANDROID_TOOLCHAIN_SUFFIX and CMAKE_<LANG>_ANDROID_TOOLCHAIN_MACHINE.
For example, the path to the linker is:
${CMAKE_CXX_ANDROID_TOOLCHAIN_PREFIX}ld${CMAKE_CXX_ANDROID_TOOLCHAIN_SUFFIX}